1. Field of the Invention
The present invention relates to a stator blade and a stator blade cascade for an axial-flow compressor such as a gas turbine, and particularly, to a stator blade and a stator blade cascade in an axial-flow compressor, in which the pressure loss in a transonic range can be reduced.
2. Description of the Related Art
There are rotor blades for an axial-flow compressor known from Japanese Patent Application Laid-open Nos. 9-256997 and 8-254156, in which a recess is formed at a substantially central location or at a location near a leading edge on the extrados (a negative pressure surface) of a blade profile, so that two shock waves are generated in a transonic range to inhibit the separation of a boundary layer, thereby providing a reduction in pressure loss. There is a blade profile applicable to both of a compressible fluid and an incompressible fluid, which is known from U.S. Pat. No. 5,395,971, in which a recess is formed at a substantially central location on each of the intrados (a positive pressure surface) and an extrados (a negative pressure surface), so that a laminar flow boundary layer region is kept long and inhibited from being separated, thereby providing an enhancement in performance at a high attack angle.
In addition, there is a rotor blade cascade for an axial-flow compressor known from Japanese Patent Application Laid-open No. 11-13692, which is designed so that the generation of a shock wave between blades is moderated by defining the distance between the intrados and extrados of adjacent rotor blades in a range of 5% from the hub of the rotor blade. Further, there is a blade profile applicable to both of a compressible fluid and an incompressible fluid, which is known from U.S. Pat. No. 5,395,071, in which a recess is formed at a substantially central location on each of intrados (a positive pressure surface) and an extrados (a negative pressure surface), so that a laminar flow boundary layer region is kept long and inhibited from being separated, thereby providing an enhancement in performance at a high attack angle.
If the flow entering the stator blade of the axial-flow compressor reaches a critical mach number, the flow speed reaches a sonic speed on the extrados of the stator blade to generate a shock wave. For this reason, a large wave drag or compressibility drag is produced to cause a reduction in performance. Therefore, to provide an enhancement in performance of the axial-flow compressor, it is necessary to moderate the shock wave generated on the extrados of the stator blade to reduce the wave drag.
Accordingly, it is an object of the present invention to provide a stator blade and a stator blade cascade for an axial-flow compressor, wherein the wave drag due to the generation of a shock wave in the transonic speed range can be suppressed to the minimum.
To achieve the above object, according to a first aspect and feature of the present invention, there is provided a stator blade for an axial-flow compressor, having an intrados producing a positive pressure and an extrados producing a negative pressure, the stator blade being disposed in an annular fluid passage, both of the intrados and extrados being on one side of a chord line, characterized in that the stator blade includes a first bulge and a second bulge on the intrados at locations on the side of a leading edge and on the side of a trailing edge, respectively.
According to a second aspect and feature of the present invention, in addition to the first feature, there is provided a stator blade for an axial-flow compressor, characterized in that the distance Xa from the leading edge to a front end of the second bulge is in a range of 0.60 less than Xa/C less than 0.90 with respect to a chord length C.
According to a third aspect and feature of the present invention, in addition to the second feature, there is provided a stator blade for an axial-flow compressor, characterized in that the distance Xb from the leading edge to a rear end of the first bulge is in a range of 0.05 less than Xb/C less than 0.40 with respect to a chord length C.
With the first to third features, when the fluid flows to the stator blade disposed in the annular fluid passage, the separation of a boundary layer is produced positively by the first bulge provided on the intrados on the side of the leading edge, whereby the generation of a shock wave on the extrados of the stator blade adjacent the intrados can be moderated to reduce the wave drag. A small increase in frictional drag is produced due to the separation of the boundary layer at the first bulge, but this increase is by far smaller, as compared with a decrease in the wave drag produced by the moderation of the generation of the shock wave and hence, the drag on the entire stator blade can be reduced substantially. The boundary layer rendered unstable by the first bulge at the leading edge of the intrados can be stabilized again by the second bulge at the trailing edge of the intrados and hence, the increase in frictional drag due to the separation of the boundary layer on the intrados can be suppressed to the minimum.
In addition, the above-described effect can be exhibited particularly satisfactorily by setting the distance Xa from the leading edge to the front end of the second bulge in the range of 0.60 less than Xa/C less than 0.90 with respect to the chord length C and by setting the distance Xb from the leading edge to a rear end of the first bulge in the range of 0.05 less than Xb/C less than 0.40 with respect to the chord length C.
To achieve the above object, according to a fourth aspect and feature of the present invention, there is provided a stator blade cascade for an axial-flow compressor, comprising a large number of stator blades disposed in an annular fluid passage, each the stator blade having an intrados producing a positive pressure and an extrados producing a negative pressure, characterized in that a distribution of distances in a chord-wise direction between the intrados of one of two adjacent stator blades and the extrados of the other of the adjacent stator blades increases from a leading edge toward a trailing edge and reaches a maximum value; then decreases and reaches a minimum value; and then increases again.
According to a fifth aspect and feature of the present invention, in addition to the fourth feature, there is provided a stator blade for an axial-flow compressor, characterized in that the distance is a length of a perpendicular line drawn from the intrados of the one stator blade to the extrados of the other stator blade.
According to a sixth aspect and feature of the present invention, in addition to the fourth feature, there is provided a stator blade for an axial-flow compressor, characterized in that the flow on the extrados of the stator blade is stabilized in a region where the distance assumes the maximum value.
According to a seventh aspect and feature of the present invention, in addition to the fourth feature, there is provided a stator blade for an axial-flow compressor, characterized in that the flow on the intrados of the stator blade is stabilized in a region where the distance assumes the minimum value.
According to an eighth aspect and feature of the present invention, in addition to the fourth feature, there is provided a stator blade for an axial-flow compressor, characterized in that the ratio of the chord length of the stator blade to the distance between adjacent stator blades is in a range of 1.5 to 3.0.
With the fourth to eighth features, by rendering unstable a boundary layer on the intrados in the region where the distance between the intrados and extrados of the stator blade cascade assumes the maximum value to positively separate the boundary layer, the generation of a shock wave on the extrados opposed to the boundary layer rendered unstable can be inhibited to reduce the wave drag. A small increase in frictional drag is produced due to the separation of the boundary layer on the intrados. However, such increase is by far smaller, as compared with a reduction in the wave drag caused by the moderation of the generation of the shock wave, and hence, the overall drag can be reduced substantially. In addition, the distance between the intrados and the extrados in the stator blade cascade reaches the maximum value and then decreases down to the minimum value and hence, by throttling the flow to accelerate it again in the region where the distance assumes the minimum value, the boundary layer can be stabilized to inhibit the promotion of the separation, thereby inhibiting an increase in frictional drag due to the separation of the boundary layer on the intrados.
The distance between the intrados and the extrados in the stator blade cascade can be defined appropriately as a length of a perpendicular line drawn from the intrados of one stator blade to the extrados of the other stator blade. Further, the above-described effect can be exhibited particularly satisfactorily by setting the ratio of the chord length of the stator blade to the distance between adjacent stator blades in a range of 1.5 to 3.0.